Volcanogenic pollution by acid water discharges along Ciwidey River, West Java (Indonesia)

Active and recently extinct volcanoes surrounding the catchment area of the upper Citarum River (West Java) are the source areas of surface waters with high concentrations of natural contaminants. Ciwidey River, one of its tributaries, originates on the flanks of Patuha, a Quaternary volcano which contains an acid (pH <1) crater lake having high contents of sulphur and chlorine, and where various other hydrothermal manifestations are active. We present the results of a geochemical study carried out in 1995-1996 to assess the impact of volcanogenic pollution along this river, whose natural flow is disturbed by the use of water for irrigation and other purposes. Concentrations of dissolved major and trace elements in the Ciwidey River show peak levels associated with the influx of acid streams. Downstream decreasing trends are attributable to dilution and other attenuation processes commonly observed in acid mine drainage. The main contaminating streams originate from two different sources: (1) a flank spring producing water which is suspected to contain a portion of leaking acid lake water; and (2) artificial sulphur-mud deposits which are the remains of sulphur production from crater-lake sediments that were exploited in the first half of this century. Fluctuations in pH and dissolved metals observed along the river during repeated sampling campaigns are correlated with seasonal successions of wet and dry periods, but the interference in the drainage pattern has modified the original natural variations. The river water shows lowest pH values and remains acid along the greatest distance during the rainy season. Downstream decreases in acidity, which are largely induced by dilution with neutral water, determine to a significant extent whether potentially harmful elements remain dissolved or are removed by precipitation or sorption onto solid phases. The seasonal variability in discharges and acid input, combined with human interference in the river flow thus creates highly dynamic conditions for mobilisation and fixation of the volcanogenic pollutants. Among the elements studied, aluminium, arsenic, boron, iron and manganese reach concentrations which deserve attention in view of the potential consequences for the human environment. In general, the volcanogenic pollutants return to baseline levels before the onfluence with the Citarum River at some 30 km from the focal point of the acid discharges. It is expected that only the most conservative dissolved elements could cause detectable changes resulting from the influx of the Ciwidey River water. However, ephemeral peak discharges cannot be excluded, and may have an impact which remained unnoticed during our sampling campaigns. © 1998 Elsevier Science B.V. All rights reserved.